Weak Intermolecular CH···N Hydrogen Bonding: Determination of CH-N Hydrogen-Bond Mediated Couplings by Solid-State NMR Spectroscopy and First-Principles Calculations.

Weak hydrogen bonds are increasingly hypothesized to play key roles in a wide range of chemistry from catalysis to gelation to polymer structure. Here, N/C spin-echo magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR) experiments are applied to "view" intermolecular CH···N hydrogen bonding in two selectively labeled organic compounds, 4-[N] cyano-4'-[C] ethynylbiphenyl () and [N,C]-2,4,6-triethynyl-1,3,5-triazine (). The synthesis of , is reported here for the first time via a multistep procedure, where the key element is the reaction of [N]-2,4,6-trichloro-1,3,5-triazine () with [C]-[(trimethylsilyl)ethynyl]zinc chloride () to afford its immediate precursor [N,C]-2,4,6-tris[(trimethylsilyl)ethynyl]-1,3,5-triazine (). Experimentally determined hydrogen-bond-mediated couplings (4.7 ± 0.4 Hz () and 4.1 ± 0.3 Hz ()) are compared with density functional theory (DFT) gauge-including projector augmented wave (GIPAW) calculations, whereby species-independent coupling values (29.0 × 10 kg m s A () and 27.9 × 10 kg m s A ()) quantitatively demonstrate the couplings for these "weak" CH···N hydrogen bonds to be of a similar magnitude to those for conventionally observed NH···O hydrogen-bonding interactions in uracil (: 28.1 and 36.8 × 10 kg m s A). Moreover, the GIPAW calculations show a clear correlation between increasing (and ) coupling and reducing C(H)···N and H···N hydrogen-bonding distances, with the Fermi contact term accounting for at least 98% of the isotropic coupling.